In the petroleum industry, wells are drilled into subsurface formations to recover hydrocarbons contained therein. A single formation typically contains a wide variety of hydrocarbons, including gaseous hydrocarbons such as methane and liquid hydrocarbons such as octane. These hydrocarbons are often associated in the formation with nonhydrocarbons such as sand, water and carbon dioxide. The hydrocarbon and nonhydrocarbon components of the subsurface formation are produced as a wellstream which flows to the surface through the well.
Before the hydrocarbons being produced by the well can be marketed, they must generally be separated from the other components of the wellstream. In addition, the liquid hydrocarbons must generally be separated from the gaseous hydrocarbons due to the different handling requirements and end uses for each. The resulting liquid hydrocarbon stream is referred to as a crude oil stream and the resulting gaseous hydrocarbon stream is referred to as a natural gas stream. Often, the crude oil stream and natural gas stream are introduced into separate common carrier pipelines for transportation to shipping facilities, refineries, chemical plants or sites of commercial and residential use. Before the crude oil and natural gas from the well can be introduced into their respective common carrier pipelines, they must first meet certain standards established by the common carrier. For example, the crude oil may be permitted to contain no more than a very small percentage of water and particulates, and the natural gas may be permitted to contain only a very small percentage of water vapor. These restrictions are aimed at keeping relatively uniform streams of crude oil and natural gas in the pipelines.
Even in the absence of common carrier requirements, practicalities often necessitate separate and relatively uncontaminated streams of crude oil and natural gas. For example, crude oil produced from offshore platforms is commonly loaded onto tankers for transportation, and natural gas is commonly sent via dedicated underwater pipelines to facilities on land. If water is being produced along with the crude oil, it is usually desirable to dispose of the water before the crude oil is loaded onto the tanker, so that tanker capacity is not wasted. Likewise, if substantial quantities of carbon dioxide are being produced along with the natural gas, it is usually desirable to separate the carbon dioxide from the natural gas so that the carbon dioxide can be disposed of at the offshore platform, thereby reserving pipeline capacity for transportation of the natural gas alone.
For these reasons and others, much effort and expense has gone into the development of systems which separate wellstream components. Many separation systems take advantage of the immiscibility and difference in densities of the various wellstream components, and make use of the force of gravity to provide the desired separation. For example, wellstreams are commonly introduced into separators, which are basically large tanks which have outlet lines for the various components at different vertical positions. After the wellstream is introduced into the separator, it is allowed to sit for a substantial period of time so that the force of gravity can cause the heavier components to settle to the bottom. Typically, the wellstream will thereby be separated into a water fraction at the bottom of the separator, a crude oil fraction at the middle and a gaseous fraction at the top. Due to the time it takes for gravity to achieve this separation, separators are usually quite large and very heavy when full.
While the force of gravity can be sufficient to separate water, crude oil and gas from one another, it is generally not sufficient to separate nonhydrocarbon gases from natural gas. To achieve this kind of separation, the gas fraction from the separator is commonly sent into a separation system which relies on differences in the physical and/or chemical properties of the various gaseous components. Systems which rely on differences in physical properties generally cause one or more of the gaseous components to separate from the remainder via liquefaction, which is induced by pressure and cooling. The compressors and heat exchangers used by such systems are usually large, heavy and expensive. Separation systems which rely on differences in the chemical properties of the various gaseous components, such as glycol separation systems, are also generally large, heavy and expensive.
The cost of the processing facilities needed to separate the components of wellstreams coming from one or more wells can constitute a major portion of the expense needed to bring a petroleum field into production. In the case of offshore petroleum fields, this problem is compounded by weight and space limitations. The more space the processing facilities require, and the more they weigh, the greater the expense required to design, construct, and deploy an offshore platform with the size and strength needed to support the facilities. For these reasons and others, it would be highly advantageous to have a separation system which is smaller, lighter and less expensive than those which are currently in existence. The present invention is aimed at providing such a system.